With the development of Internet technologies in recent years, telephone services enabled by making use of a network (IP network) on the basis of Internet Protocol (IP) have been proposed in addition to the conventional telephone services on the basis of the circuit switching system enabled by making use of a circuit switching network (public switched telephone network PSTN).
In the case of such a telephone service making use of a packet network, a communication path connectable to a telephone terminal (including a facsimile terminal and the like) in a predetermined area of the packet network is set by the use of a connection control device, i.e., a so-called call agent (MGC), which is associated with the predetermined area (referring, for example, to “A. Arango, A. Duran, I. Elliott, C. Huitema, S. Pickett, Media Gateway Control Protocol (MGCP) Version 1.0, IETF RFC2705, October 1999”).
Also, the telephone terminal can transmit and receive packet signals by being connected with a Voice over IP (VoIP) conversion unit which serves to perform conversion between voice band signals and packet signals. Furthermore, the VoIP conversion unit is provided with a port through which it is connected to the packet network for transmitting and receiving packet signals, and a port through which it is connected to a circuit switching network for transmitting and receiving voice band signals. A telephone terminal connected to such a VoIP conversion unit can make use of both the packet network and the circuit switching network.
Furthermore, with the advent of the wide bandwidths of communication networks, xDSL modems are widely distributed to realize high speed digital communications by the use of copper telephone cables in a subscriber line area. Along with the release of the telephone service making use of a packet network and the popularization of xDSL modems, xDSL modems serving also as a VoIP conversion unit as described above have been provided.
Also, the packet network and the circuit switching network as described above are connected through a gateway device which performs the conversion of communication signals and the conversion of call control procedure (signaling protocol). It is possible to provide a communication link between a telephone terminal connected to the packet network and a telephone terminal connected to the circuit switching network, i.e., a public switched telephone network by connecting the packet network and the circuit switching network.
However, a connection request for setting a communication path between telephone terminals is issued from the caller telephone terminal, when there is a failure to perform the operation of the port of a VoIP conversion unit through which the packet network is connected in the destination side or the operation of a connection control device serving to set a communication path to the VoIP conversion unit in the destination side, then there is a problem that the connection request cannot be granted even though the communication path could be set between the telephone terminals through the packet network and the circuit switching network.
The present invention is invented taking into consideration the above circumstances. It is an object of the present invention to provide a communication path setting method and a communication path setting system capable of preventing call loss by judging the operating condition of a VoIP conversion unit or a connection control device, and making use of a circuit switching network in cooperation with the packet network in order to set a communication path therethrough in the case where it is difficult to set a communication path to the destination telephone terminal only through a packet network.